Ultrasound is used for many purposes including the detection of defects in materials, thickness and corrosion monitoring, acoustic and mechanical properties measurements, fluid level measurements, remote sensing of objects through air, particulate measurements in fluids, and so on. The significance of ultrasound in such applications, particularly on solid materials, is that the test are entirely nondestructive in nature. Furthermore, few or no sample preparations are required and in general access to only one side of the test sample is sufficient.
The principle of ultrasound in such applications is simple. Certain materials, known as piezoelectric materials, emit electricity when mechanical waves are applied to their surfaces. Conversely, these materials emit mechanical or ultrasonic waves when an electrical voltage is applied on their faces. The frequency of waves so generated is determined by a given piezoelectric material's thickness and its frequency constants. Ultrasound waves from a piezoelectric material (properly housed inside a transducer device) when introduced into materials, behave much like the light waves. That is, they are reflected, refracted, diffracted, transmitted and so on. If the propagating ultrasonic beam encounters a physical discontinuity, such as a flaw, pore, crack, or a gross chemical heterogeneity, a portion of ultrasonic energy is reflected back to the piezoelectric material by such a discontinuity. The rest of the ultrasonic energy continues to propagate in the material until it meets the end of the material, generally its far side. All this information may be displayed on an oscilloscope screen in the form of reflected or transmitted signals. From this information, sound velocity, presence or absence of defects, thickness or corrosion of the materials, for example can be estimated.
Ultrasonic techniques are thus used in a wide variety of industries. The industrial functions served by ultrasonic nondestructive methods include quality control, quality assurance, product control, process control, research and development of materials. The major implications of ultrasonic uses are safety in materials application and economy in the materials manufacture.
Heretofore, the environment in which ultrasonic testing was performed was generally confined to near room temperature (20.degree. C.) and pressure conditions. However, it is common knowledge that manufacture and applications of most materials involve elevated temperatures. Therefore, in order to monitor the conditions of the materials in situ it is desirable to be able to conduct ultrasonic testing at elevated temperatures.
Ultrasonic testing equipment is essentially composed of two parts: the transducer device, and the electronic instruments. While the electronic instrument can be kept away from the extremities of the test environment, the transducer, the most critical and important part of the ultrasonic testing system, often is in contact with the test part. Thus, the transducer must withstand the conditions of test part; it must perform satisfactorily at high temperatures.
Designs for ultrasound transducers are legend. The structure often comprises a case for supporting a piezoelectric element sandwiched between a wear plate and a damping material. See for example U.S. Pat. No. 3,376,438.
The current commercial transducers of which applicant is aware cannot satisfactorily withstand elevated temperatures. Their use is generally confined to -10.degree. to 70.degree. C. Transducer designs to withstand elevated temperatures in the neighborhood of 200.degree. C. either utilize complex cooling systems or sound transfer bars that space the transducer from the hot surface of material. Both approaches have drawbacks: cooling is unreliable and therefore the transducer is subject to degradation. Space bars complicate the interpretation of data and attenuate the ultrasound. Further, none of these approaches can be used on hot surfaces on a continuous basis.
A reason for the unsuitability of commercial transducers when used at elevated temperatures is that various materials and mechanical designs used in the transducer device are not sufficiently temperature resistant. One such material in all commercial transducers is an organic polymer. An organic polymer, in the form of epoxies, rubbers, etc., is used for bonding, potting, and sealing of the transducer components. Even the best commercially available organic polymers cannot withstand temperatures beyond 250.degree. C. Moreover, in the presence of certain acoustic coupling liquids, the organic polymers are even less stable on hot surfaces. Even if the best available materials were used with existing mechanical designs, the transducers would not be sufficiently temperature resistant.
The piezoelectric element is usually but not always a circular disc of a thickness corresponding to its resonant frequency. It usually possesses metallized layers on both faces, such as the ones obtained by thick film or thin film deposition techniques. The damping material is generally composed of heavy metal powder, such as tungsten, molybdenum, nickel, copper, iron, chromium, etc. immersed tightly in an organic polymer, such as polyurethane or other rubber like materials. The damping may utilize, either a single metal powder, or it may use the combination of several powders. The damping material provides two important functions: it helps in minimizing the excessive resonance (ringing) of the piezoelectric material and it eliminates spurious ultrasonic reflections that may be generated by the structure of the damping itself.
Typically, in the case of hard faced contact transducer devices the damped piezoelectric material is bonded to a metallized hard wear face with an organic polymer, such as an epoxy. The purpose of the wear face is to protect the critical piezoelectric material, which in general is a mechanically weaker material. The commonly used wear faces are dense alumina, tungsten carbide, or any other super-hard materials.
In order to apply electrical voltage to the transducer, a ground lead is soldered on a metallized surface on the piezoelectrical material in contact with the wear face material. The positive electrical lead is often taken from the top face of the piezoelectric material that is in contact with the damping material.
The above assembly is secured inside an appropriate case by a potting material. The case material is generally steel. The potting material is generally an organic polymer, such as an epoxy.
After all the bonding, potting, and sealing compounds have been cured, the positive and ground leads are either connected to a co-axial cable, or they are terminated directly on the casing with a high frequency co-axial connector.